diff --git a/src/RFID.cpp b/src/RFID.cpp
index 03b40d5..1c4eb2a 100644
--- a/src/RFID.cpp
+++ b/src/RFID.cpp
@@ -161,7 +161,8 @@ bool RFID::writeBlock(MFRC522Constants::Uid &uidToRead, byte blockAddr, uint8_t
 		return false;
 	}
 
-	byte size = sizeof(comData);
+	// Take into account the two bytes used only for the read commands.
+	byte size = sizeof(comData) - 2;
 	for (byte& bufferByte: comData)
 		bufferByte = 0;